calculating forces
 

Hi all
I saw in forces code that shear stress is calculated by:

shear stress = - mu*gama

in code:
-rho()*nu*dev(twoSymm(fvc::grad(U)))

what is the minus in this formula?
Now I want to calculate forces for viscoelastic fluid. should I write:

total shear stress = -tau - mu*gama

that tau is polymeric stress.

Thanks